Pharmacological responses to the intracerebral administration of nicotine

Rethinking the Conditions and Mechanism for Glymphatic Clearance

Critical studies that form the foundation of the glymphatic system and the clearance of metabolic by-products of unwanted proteins from the brain are reviewed. Concerns are raised about studying glymphatic flow in anesthetized animals and making assumptions about the whole brain based upon data collected from a cranial window on the cortex. A new model is proposed arguing that the flow of cerebral spinal fluid and parenchymal clearance in the perivascular system of unwanted proteins is regulated by circadian changes in brain temperature and blood flow at the level of the microvasculature.

Fluctuations in central and peripheral temperatures induced by intravenous nicotine: central and peripheral contributions

Nicotine (NIC) is a highly addictive substance that interacts with different subtypes of nicotinic acetylcholine receptors widely distributed in the central and peripheral nervous systems. While the direct action of NIC on central neurons appears to be essential for its reinforcing properties, the role of peripheral actions of this drug remains a matter of controversy. In this study, we examined changes in locomotor activity and temperature fluctuations in the brain (nucleus accumbens and ventral tegmental area), temporal muscle, and skin induced by intravenous (iv) NIC at low human-relevant doses (10 and 30μg/kg) in freely moving rats. These effects were compared to those induced by social interaction, an arousing procedure that induces behavioral activation and temperature responses via pure neural mechanisms, and iv injections of a peripherally acting NIC analog, NIC pyrrolidine methiodide (NIC-PM) used at equimolar doses. We found that NIC at 30μg/kg induces a modest locomotor activation, rapid and strong decrease in skin temperature, and weak increases in brain and muscle temperature. While these effects were qualitatively similar to those induced by social interaction, they were much weaker and showed a tendency to increase with repeated drug administrations. In contrast, NIC-PM did not affect locomotion and induced much weaker than NIC increases in brain and muscle temperatures and decreases in skin temperature; these effects showed a tendency to be weaker with repeated drug administrations. Our data indicate that NIC's actions in the brain are essential to induce locomotor activation and brain and body hyperthermic responses. However, rapid peripheral action of NIC on sensory afferents could be an important factor in triggering its central effects, contributing to neural and physiological activation following repeated drug use.

Menthol and nicotine oppositely modulate body temperature in the rat

Menthol is a prominent additive in many tobacco products. To investigate possible interactions with nicotine, (-)-menthol (200 or 400 mg/kg) and (-)-nicotine (0.5 mg/kg) were injected subcutaneously in rats, and body temperature, which is modulated by brain nicotinic acetylcholine receptors, was measured. Nicotine caused robust (-1.6 degrees C) hypothermia, the magnitude and time course of which was not altered by menthol pretreatment. Menthol alone produced mild (0.4-0.8 degrees C) hyperthermia, which was not secondary to locomotor activation. Nicotine and menthol influence body temperature independently and oppositely; menthol does not appear to influence the function of the central nicotinic receptors that control body temperature.

The impact of smoking on panic disorder: an initial investigation of a pathoplastic relationship

A disproportionate number of persons with panic disorder (PD) smoke cigarettes compared to individuals with other anxiety disorders and people in the general population. Currently, there is little theoretical or empirical understanding as to how smoking impacts those with PD. The present descriptive psychopathology study provided a test of the pathoplastic relationship between smoking and PD by investigating the additive negative impact of smoking on PD in terms of emotional, functional, and physical health impairment indices. Patients with PD who regularly smoke (PDSM) were compared to PD alone (N=122) in a cross-sectional analysis. As expected, persons with PDSM reported significantly more severe and intense anxiety symptoms and social impairment compared to persons with PD alone. No differences were evident for panic attacks or physical illness variables. These findings are discussed in relation to Pathoplastic models of dysfunction.

Nicotinic receptor agonists exhibit anxiolytic-like effects on the elevated plus-maze test

The effects of nicotinic receptor agonists on the elevated plus-maze test of anxiety were investigated in CD1 mice after intraperitoneal injections. Nicotine and lobeline, but not cytisine, exhibited a significant increase in the time spent by the mice in the open arms, a measure of anxiolytic activity. Nicotine also increased the total number of arm entries, a measure of general activity, but this effect was secondary to its anxiolytic-like properties. Nicotinic receptor antagonists on their own did not modify the behavior of mice in the maze. The effect of nicotine was mediated by central nicotinic receptors as it was blocked by the centrally-acting nicotinic antagonists mecamylamine and chlorisondamine, but not by hexamethonium (a peripherally acting blocker). Cotinine, the major metabolite of nicotine, was evaluated at different times after systemic injections and had no effect in the plus-maze. The anxiolytic-like profile induced by nicotinic receptor stimulation was not associated with potentiation of alcohol effects, a liability associated with the benzodiazepine therapy. This study demonstrates the anxiolytic-like properties of nicotine and lobeline in mice, and suggests that central nicotinic receptors are involved in the expression of emotional behavior.

Nicotinic receptor agonists facilitate retention of avoidance training: participation of dopaminergic mechanisms

The effect of nicotinic receptor agonists on retention of the inhibitory avoidance (IA) response were investigated in mice. Animals received intraperitoneal drug injections before training, and retention was evaluated 24 h later. Nicotine and cytisine, but not lobeline, significantly increased retention of the IA training. Cotinine, the main metabolite of nicotine, was inactive in the same test. Retention was not affected by the injection of the D1-D2 receptor antagonist cis-flupentixol, but the preadministration of cis-flupentixol significantly blocked the facilitatory effect of nicotine and cytisine on memory. These results demonstrate that the nicotinic receptor agonists nicotine and cytisine facilitate the retention of avoidance responses and suggest that this effect is mediated through central dopaminergic pathways.

Effect of acute and subchronic nicotine treatment on cortical acetylcholine release and on nicotinic receptors in rats and guinea-pigs

1. The effect of acute and chronic (16 days) administration of nicotine on cortical acetylcholine (ACh) release, gross behaviour and brain nicotinic binding sites was investigated in rats and guinea-pigs. 2. The drug, injected either subcutaneously (0.45-0.90 mg kg-1) or intracerebroventricularly (1, 3 and 5 micrograms) increased the cortical ACh release, in a dose-dependent manner, through mecamylamine-sensitive receptors for 1-2 h in both species. 3. Chronic treatment significantly increased basal ACh release in the rat and slightly lowered it in the guinea-pig, but the response to a challenging dose of nicotine was proportionally maintained in both species. 4. The number of nicotinic receptors was four times higher in the rat than in the guinea-pig and was not dependent on the radioligand used ([3H]-nicotine or [3H]-ACh, in the presence of atropine) to determine this. The nicotinic binding sites showed an apparent increase in chronically treated rats but no change in guinea-pigs. 5. Tolerance to the inhibitory effect of the drug, assessed with the T maze test, was found in the rat. No apparent change in gross behaviour was detected in the guinea-pig. 6. It is concluded that chronic nicotine treatment causes evident tolerance to its inhibitory effect on behaviour in the rat, but no adaptation to its excitatory properties on the cholinergic brain structures in rats and guinea-pigs.

Neuroendocrine actions of nicotine and of exposure to cigarette smoke: medical implications

Over many years a large number of studies have demonstrated that nicotine and exposure to cigarette smoke produce marked neuroendocrine changes in animals and in man. The initial effects of nicotine are characterized by a marked hypersecretion of ACTH, vasopressin, beta-endorphin, prolactin and LH. Many of these very acute stimulatory effects of nicotine rapidly disappear, probably due to a desensitization of the central nicotinic cholinergic receptors involved. Instead, upon acute intermittent treatment with nicotine or exposure to cigarette smoke, an inhibition of prolactin, LH and TSH secretion occurs, which is associated with maintained hypersecretion of corticosterone. These effects are probably mediated via activation of central cholinergic receptors of the ganglionic type. Evidence indicates that the inhibitory effects of nicotine on LH and prolactin secretion are produced via an activation by these nicotinic receptors of the tubero-infundibular dopamine neurons, releasing dopamine as a prolactin inhibitory factor. Dopamine inhibits LHRH release via an axonic interaction involving D1-like dopamine receptors in the median eminence. It therefore seems possible that the reduced fertility found in heavy smokers may be counteracted by D1 receptor antagonists. The symptoms associated with glucocorticoid hypersecretion induced by nicotine is discussed considering not only the peripheral side effects but also permanent deficits in hippocampal glucocorticoid receptors and loss of hippocampal neurons. In view of the important influence of hormones on immune functions, it seems likely that smoking will cause disturbances in immune responsiveness. Finally, the nicotine-induced alterations of neuroendocrine function, especially in the pituitary-adrenal axis and in vasopressin release, may also lead to behavioural consequences in smokers, especially in the withdrawal phase.

Neuroregulatory effects of nicotine

The impact of nicotine on the central nervous system is, in an important sense, neuroregulatory, with cascading effects on physiological and biochemical function as well as on behavioral activity. Accordingly, the neurotransmitter and neuroendocrine effects of nicotine constitute a critical part of its biological action, which includes reinforcing as well as pathophysiological consequences. This review focuses on nicotine's effects on cholinergic and non-cholinergic nicotine receptors and on the responses of catecholamines, monoamines, hypophyseal hormones, and cortisol. The contribution of critical variables, such as timing and duration of neuroregulator release and the patterns that make up the total response, is still largely unknown, particularly with regard to the effects of environmental context, history of nicotine use, and mode of administration. The evidence suggests that by altering the bioavailability of the above-listed neuroregulators, nicotine serves as a pharmacological "coping response", providing immediate though temporary improvement in affect or performance in response to environmental demands. Much of what is known to date is based on studies involving the administration of agonists and antagonists under different environmental conditions. Newer technological approaches such as autoradiography and positron emission tomography show potential for determining the neuroregulatory patterns involved and specifying nicotine's locus of action relevant to its behavioral and physiological effects.

The effects of long-term nicotine treatment on locomotion, exploration and memory in young and old rats

To assess the effects of long-term treatment with nicotine on several behavioral measures (locomotor activity, exploratory efficiency, habituation, short-term and long-term memory) of young (5 months) and old (22 months) rats in a hexagonal tunnel maze, nicotine was added to the drinking water (0, 20 or 50 mg/l) for up to 131 experimental days. With the exception of effects on exploratory efficiency, young and old rats did not differ in their response to the drug. Nicotine decreased body weight throughout the experiment. Nicotine treatment reduced water intake during the first 30 min of the daily 4.5 h access to drinking water. Nicotine increased locomotor activity throughout the experiment. When nicotine treatment was discontinued during a 7-day withdrawal period, locomotor activity immediately dropped to control values. Intertrial habituation was not affected by nicotine. Long-term nicotine treatment had an attenuating effect on exploratory efficiency in young rats; however, the drug did not influence performance in tasks measuring spatial memory. Finally, age increased weight, decreased locomotor activity and impaired exploratory efficiency and short-term memory. Age, however, did not affect the performance of the long-term memory task.

Evidence for an olfactory receptor which responds to nicotine--nicotine as an odorant

The tobacco alkaloid (S)(-)-nicotine, when applied as a vapour to an in vitro head preparation, stimulates the olfactory epithelium in three strains of rats and to a lesser extent in two strains of mice. The electro-olfactogram (EOG) generated by nicotine has similar characteristics to the EOGs produced by known odorants. The nicotine EOG increases with increasing concentration of nicotine vapour (1-100 nM) applied to the olfactory epithelium. Differential reduction of the nicotine EOG by the lectin concanavalin A is seen in Wistar and Lister Hooded rats. The reduction of the nicotine EOG by concanavalin A is prevented by adding alpha-methyl-D-mannoside to the lectin superfusion medium. This suggests that there is a glyco-moiety associated with at least one olfactory receptor responding to nicotine. Our results suggest that rat olfactory epithelium has receptor sites for nicotine. Nicotine is an unusual compound because it shows both odorant and pharmacological properties.

Effects of nicotine on beta-endorphin, alpha MSH, and ACTH secretion by isolated perfused mouse brains and pituitary glands, in vitro

The effects of nicotine on secretion of the pituitary peptides beta-endorphin, alpha MSH, and ACTH were studied using the isolated perfused mouse brain (IPMB) and isolated superfused pituitaries of C3H mice. Nicotine (6.1 microM) stimulated secretion of beta-endorphin immunoreactivity from C3H IPMB approximately twofold. Secretion of alpha MSH immunoreactivity was stimulated approximately two- and sixfold by 6.1 microM and 12.2 microM nicotine, respectively. However, nicotine (6.1 microM) had no direct effect on the secretion of beta-endorphin, alpha MSH, or ACTH immunoreactivities from the isolated superfused pituitaries. The data suggest nicotine acts in the brain to stimulate pituitary secretion of alpha MSH and beta-endorphin. Electrocorticographic (ECoG) activity of the IPMB was monitored. Nicotine induced characteristic ECoG changes including a reduction of input voltage, a biphasic response of rapid desynchronization followed by prolonged synchronization, and seizure at high doses (12.2 microM).

Isolation of a nicotine binding site from rat brain by affinity chromatography

With the use of affinity chromatography, a [3H]-nicotine binding site was purified almost 1,000-fold from a Triton X-100-solubilized extract of rat brain neural membranes. The affinity column was prepared by conjugation of (R,S)-6-(2-hydroxyethyl)nicotine to epoxy-activated Sepharose. Further purification of the material from the affinity column was resolved by using another column of the same affinity gel, resulting in the isolation of a major protein (about 95% purity) that had a Mr of 56,000, as determined by NaDodSO4/polyacrylamide gel electrophoresis, with very minor components ranging in Mr from 47,000 to 83,000. With the use of various nicotine analogues, it was shown that the purified material exhibited nearly identical binding characteristics to rat brain membrane preparations, including stereoselectivity for the nicotine enantiomers. The Kd of the purified site, 3.5 x 10(-9) M, was similar to that observed with membrane and Triton X-100-soluble preparations, whereas the binding capacity was greater than 25 pmol/mg of protein, as compared to 0.07 pmol/mg of protein in the starting material. The results are discussed in relation to the purified nicotinic cholinergic receptor from electroplax. It was concluded that the nicotine site in rat brain was different from the cholinergic receptor of electroplax or calf skeletal muscle.